Porcelain enamel article and method of making same



United States Patent PORCELAIN ENAMEL ARTICLE AND METHOD OF MAKINGSAlViE Benjamin J. Sweo, Lakewood, and Le Roy L. Moss, Cleveland, Ohio,assignors to Ferro Corporation, a corporation of Ohio No Drawing.Application April 5, 1950, Serial No. 154,188

2 Claims. (Cl. 117-70) This invention relates as indicated to porcelainenameling and has more particular reference to an enamehng procedure andarticles produced therefrom characterized by the direct application to aferrous surface of a finish coat porcelain enamel which is substantiallyfree of all adherence promoting oxides and which enamel has highersurface gloss, more uniform color and higher acid and scraitchresistance than any porcelain enamels heretofore use As indicated above,the prior art practice of insuring adequate adherence between porcelainenamel and steel has been to use adherence promoting oxides. Theseoxides are usually classified as the ground coat adherence promotingoxides such as nickel, cobalt and manganese and the finish coatadherence promoting oxides such as antimony and molybdenum. In applyinga white or light colored finish coat porcelain enamel directly to asteel Work piece the ground coat adherence promoting oxides cant be usedsince they are dark in color. The finish coat adherence promoting oxidessuch as antimony and molybdenum make enamels which are noted for theirlack of opacity, acid resistance and weathering properties. Variousmethods, other than the use of adherence promoting oxides, have beenused in an endeavor to improve the adherence of finish coat porcelainenamels to steel. One of such methods has been by the use of steels ofspecial compositions. Another has been by the use of the so-callednickel-flash step by which an attempt is made to deposit a predeterminedamount of nickel on the steel surface by galvanic action.

The use of special steels is undesirable for many reasons, the principalone being cost.

The use of the galvanic process is undesirable since that method leadstov inconsistent results, in that the nickel deposits vary in characterand amount from steel to steel or for that matter the nickel depositwill vary from one portion to the next on the same piece of steel.

As previously indicated besides the difficulty of obtaining properadherence the direct application of a finish coat porcelain enamel to asteel surface is also faced with the problem of obtaining desirablesurface characteristics of the fired enamel, uniform color and acid andscratch resistance.

In the art of porcelain enameling, the porcelain enamel frit is milledin water to a predetermined fineness in the presence of variouselectrolytes, opacifying agents and clay or clay-like materials. Theclay or clay-like materials are added to providing suspending andbinding properties to the milled porcelain enamel. Although suspendingagents of type provide suspension properties their presence in ceramicslips, which are subsequently converted to vitreous coatings, have agreat many undesirable properties. The presence of clays or the like inporcelain enamel finish coats markedly reduces gloss, changes the color,reduces the acid resistance, lowers scratch and abrasion resistance andincreases the maturing .temperature required to produce a satisfactorysurface. Furthermore, in the art of porcelain enameling as nowpracticed, the application of ceramic materials, containing clay, toiron work pieces is accompanied by gas evolution attendant with thefollowing reactions:

The water for the above reaction is supplied by the clay.

Since clay is a hydrous material, during the step of firing the bisquecoat onto the ferrous work piece the clay undergoes dehydration andliberates its water. The water thus liberated reacts with the ferrouswork piece, as illustrated above, with the liberation of hydrogen. Theliberated hydrogen in its attempt to escape forces its way through themelting porcelain enamel causing bubbles. It has been furtherascertained and is well known to those skilled in the art of porcelainenameling, that the presence of clay affects the acid resistance of thefired porcelain enamel coating. The acid resistance of fired porcclainenamels is a direct function of the amount of clay used in the millingoperation. ln other words, the more clay that is used the lower the acidresistance of the fired porcelain enamel.

It is the principal object of our invention to provide a porcelainenamel article and a method of producing the same characterized by thedirect application to ferrous work pieces of porcelain enamels which donot require the presence therein of any substantial amounts of adherencepromoting oxides and the like.

It is a further object of our invention to provide porcelain enamelarticles which are superior from the standpoint of surfacecharacteristics, scratch resistance, gloss color and acid resistance toany which have been capable of production by any well known prior artmethods.

Other objects of our invention will become apparent as thedescriptionproceeds.

To the accomplishment of the foregoing and related ends, said inventionthen comprises the features hereinafter fully described and particularlypointed out in the claims, the following description setting forth indetail certain illustrative embodiments of the invention, these beingindicative, however, of but a few of the various gays ciln which theprinciple of the invention may be emp oye Broadly stated, this inventioncomprises the method of porcelain enameling wherein the surface of theferrous article to be coated is first cleaned and roughened; a layer ofnickel deposited thereon by the chemical reduction method and then thereis applied thereto a finish coat porcelain enamel clay-free slip whichdoes not require the presence of substantial amounts of adherencepromoting oxides, said slip further characterized by the inclusiontherein of surface active agents.

It will be observed from the foregoing broad statement of our inventionthat the method which comprises such invention for the production ofarticles which are likewise new, in that they have never been madeheretofore, may be divided into three principal steps which forconvenience in the further description of our invention will be treatedin the order in which they are usually performed.

Cleaning and roughening the metal The ferrous surface to which theenamel is to be eventually applied when practicing our invention needsto have two primary characteristics. First, it must be relatively clean,that is, free from all foreign materials such as dirt, grease and theusual coat of metallic oxide. Second, the surface must be reasonablyrough for best adherence of the enamel. This preparation of the ferroussurface may be accomplished by chemical means or by mechanical means orby a combination of the two. if it is desirable to effect thepreparation of the surface by chemical means, this can be accomplishedconveniently by the use of pickling baths which leave a relatively roughsurface. The pickling'baths which are available for this purpose are,for example, those employing nitric acid and combinations of nitric acidwith other acids and baths which are primarily sulphuric acid baths, butwhich also contain nitric acid or other oxidizing agents. This type ofbath has two principal characteristics. First thebath is extremelyvigorous in its action so that the required pickling time is relativelyshort and second, this type of bath produces a surface which may beidentified, for convenience, as a deeply etched surface, in that itcontains pits which are relatively deep with the crests between suchpits relatively sharp. Because of this peculiar type of surface justdescribed which is produced by pickling baths of the type just describedthey are preferred for use in our process since it has been found thatadherence between 3 the enamel and the ferrous work piece is greatestwhen the steel surface is thus preliminarily prepared.

Instead of utilizing pickling baths of the type above de scribedsatisfactory results may be obtained by utilizing pickling baths inwhich the principal components are salts which are readily reduced inthe presence of iron, for example, trivalent iron salts such as ferricchloride. Since the ferric chloride picking process wherein theefiective components of the baths are continuously regenerated by theintroduction of chlorine thereto are well known in the pickling art, itis believed unnecessary to further describe such process at this point.

If it is desired to use mechanical means for preparing the surfaceinstead of the chemical means just described satisfactory results may besecured by a simple sand blasting operation which is effective to notonly remove the foreign material, but also leave the sheet reasonablyrough. It is within the contemplation of our invention to combinechemical and physical means for the preparation of the metal work pieceand in this the sand blasting operation which is described may bepreceded or followed by a second step and when the pickling step is lastto be employed then it should be preferably of the type which willproduce the deeply etched surface of the character previously described.

Nickel depositions The nickel flash method previously referred to, i. e.the process for the deposition of nickel which depends upon galvanicaction by the Work piece and the bath has not been satisfactory for usein our process. It is believed that one reason why deposition of thenickel by galvanic action does not give satisfactory results is not onlythe inconsistency of that process and the variations in the depositsresulting therefrom, but also the nature or character of the nickeldeposit resulting from such process. Since the very nature of theprocess is galvanic, this requires that there be a certain area of thesteel surface always exposed to the action of the bath if the galvanicprocess is to proceed. The nickel layer thus laid down by the galvanicprocess is not what may be termed a uniform or substantially continuouslayer of nickel. Instead it is a coating of nickel which is interruptedby areas wherein the base metal is exposed or only superficially coated.

It has been established that porcelain enamel deposited over a thicklayer of pure nickel has very poor adherence to such nickel. It will beconcluded therefore that for proper adherence by the enamel to the workpiece there must be a certain amount of iron in combination or elementalform available in the bonded area. We have found that the iron thusavailable on a surface coated with nickel by the galvanic method is toogreat for best results and it is believed that this accounts for thepoor bond which is secured between the base metal and the enamel if onlythe nickel formation is used and if there are not adherence-promotingoxide present in the enamel.

In order to provide a lower ratio of iron to nickel on the surface ofthe steel work piece and to bring such ratio into the range whereingreatest adherence between the enamel and the work piece is secured, itis essential that the nickel coating deposit be substantially uniformwhile at the same time permitting the occurrence of at least a minoramount in free or combined form of iron at the bond. The process ofdepositing nickel by means of an electrolytic bath will produce asubstantially continuous layer of nickel but such process isunsatisfactory for a number of reasons. First it can be usedsuccessfully only on simple shapes such as fiat sheets because if thearticle treated has any substantial contour deviating from a fiat planethere will be suflicient variation in the thickness of the nickeldeposited on the various areas of the article to produce best results.Since the nature of the nickel deposit, i. e. its thickness andcharacter are quite critical, when the article is contoured thethickness of the coat on the high points, for example, will besufliciently different from the thickness of the coat on the low pointsfor example, so that if the coatmg on one place is adjusted to thenarrower range for best results, the coating in the other area will beoutside of such range. Thus the process of depositing the nickelelectrolytically can be used satisfactorily only on fiat surfaces suchas unfabricated steel sheets and strips and even on such surfaces thecharacter of the deposit laid down by the electrolytic method issufliciently different from 4 the character of the deposits laid down bya chemical reduction method that it is not possible to secure the highorder of adherence with the electrolytic process as it is with theprocess wherein the nickel is laid down by chemical reduction.

When flat steel sheets are prepared by the electrolytic process and theyare then subsequently formed into the various shapes required by theenameling industry, such forming, especially if it is accomplished withany burnishing, grinding or welding steps will either sufficientlydisrupt the layer of deposited nickel or destroy it entirely so that theresultant work piece is defective because of unsatisfactory adherence inthe area where these operations have been performed.

Accordingly our invention comprises the coating of the work piece withnickel which is deposited thereon from a bath wherein the nickel isgenerated by chemical reduction.

As previously indicated, the amount of nickel deposited is critical forbest results. There is one feature, however, which has an influence onthe amount of nickel which will produce optimum results and that is theroughness characteristic of the steel sheet. It will be found that asthe roughness or etched characteristics of the sheet vary the amount ofnickel to be deposited will vary also. As indicated above, preferredresults are secured when the surface preparation of the work leaves itin a deeply etched condition. On this type of surface the amount ofnickel to be deposited for best results will be found to lie between0.100 and 0.200 gram of nickel per square foot of surface area. Acoating of approximately 0.150 to 0.1750 gram of nickel per square footof surface area will generally be the preferred range with best resultssecured in the neighborhood of about 0.1625 gram of nickel per squarefoot of surface area. The baths Which may be used for the chemicalreduction of nickel and its deposition onto the work are described inthe literature. These baths may be either alkaline or acid. The alkalinebaths are described in Journal of Research-National Bureau of Standards,July 1946. A typical example of an alkaline bath which may be used incarrying out our invention is as follows:

G./liter Nickel chloride 30 Sodium hypophosphite 10 Ammonium chloride 50Sodium citrate Ammonium hydroxide to a pH of 8 to 10.

The alkaline baths have one drawback in that they are, under certaincircumstances, inconvenient and uneconomical because at elevatedtemperatures there is a rapid loss of ammonium and the bath gives offcopious fumes. The acid type baths which may be used in this step of myprocess are described in the Journal of Research-- National Bureau ofStandards, November 1947. A representative example of an acid bath is asfollows:

G./liter Nickel chloride 30 Sodium hypophosphite 10 Sodium acetate 10 pH4 to 6.

Since the acid baths generally operate with a very low loss ofconstituents by vaporization this type of bath is somewhat morecontrollable than the alkaline type baths described above.

In carrying out the nickel deposition step the previously treated sheet,as prepared by step 1, of our improved process, is immersed in the bathfor a length of time on the order of about one minute at about 70 C., i.e., for a period of time depending on temperature and other conditionssuch as the condition of this bath in order to lay down a deposit ofnickel Within the range specified above. After the work is removed fromthe nickel bath it is washed, preferably in warm water, preparatory toreceiving the application of enamel which occurs in the third step ofour process. The application of the enamel comprises the third step ofour process and should be performed within a reasonable time after thecompletion of the nickel deposition. It should be noted that the surfaceof the nickel Will not change substantially from the condition in whichit was immediately after the nickel was laid down.

Enamel step It has long been recognized by those skilled in the art ofporcelain'enameling that a material and method of suspending groundporcelain enamel frit without the use of clay is' extremely desirable.We have found that by adding small amounts of a cationic surface activeagent instead of clay we can provide a porcelain enamel which when firedonto a ferrous work piece will have none of the aforementionedundesirable properties. We have also found that if the porcelain enamelslip, containing a cationic surface active agent, sets up too much or becomes too thick for spraying, anionic surface active agents can be usedto cause the slip to become thinner or less viscous. Thus we haveprovided a material and method for simply and economically controllingthe set of a porcelain enamel slip.

In the preferred embodiment of the present invention we use watersoluble surface active agents. We have found that by using the watersoluble type we can more readily control the set or loss of set of theporcelain enamel slips. However, it is to be noted that the waterinsoluble types can be used. In combination with the surface activeagents we can use any of the well known mill additions such as potassiumchloride, potassium carbonate, sodium nitrite, etc. to eliminate tearingof the fired piece and sodium aluminate, potassium fluoride, etc. toobtain desired hardness of the bisque coat.

The following table gives a partial list of cationic surface activeagents which we have found to be particularly useful in the presentinvention:

Trade Name Chemical Composition Emcol 888 Alkyl-aryl pyridiuiumchloride. Triton K-GO Alkyl dimethyl benzyl ammonium chloride. AlroQuaternary O Higllf 511010011181 weight imidazolinlum c on e. AlkatergeC Substituted oxazoline.

Table Il gives a partial list of anionic surface active agents which areuseful in thinning an enamel slip with too high a set.

While in the preferred embodiment of our invention we use cationicsurface active agents of the type shown above, we have found that avolatile organo halogenosilane such as methyl chloro-silane can also beused as an agent to cause particles of milled frit from settling.Porcelain enamel frit is ground dry in a conventional ball mill in amoisture laden atmosphere containing a small percentage of methylchloro-silane. Under the condition of grinding, the frit particles arereduced to the desired size and at the same time acquire a surfacecondition which renders them water repellant. The frit thus milled isthen mixed with water by agitation to a suspension having the desiredproperties for application to metal by any of the accepted applicationtechniques. The suspension so prepared shows little or no tendency forthe suspended frit particles to settle out over extended periods oftime. If a condition arises wherein it is necessary to lower the set ofthe suspended frit in water the anionic surface active agents, used inthe preferred embodiment of our invention can be used.

We have found that only a small amount of cationic surface active agentis necessary to give excellent suspension of the ground frit in water.It is to be understood that the amouut of surface active agent used isbased on the active ingredients, since the surface active agents containwater or other inert ingredients. In other words, if it is necessary touse 0.39% of alkyl dimethylbenzyl ammonium chloride to get proper set itwould be necessary to actually add 1.56% of the Triton K-60 L3 in orderto get 0.39% of active ingredients in the enamel slip. We havesuccessfully used 3% cationic surface active agent (based on the activeportion of the cation c surface active agent). However, from an economicstandpoint we prefer using from 0.001% to about 0.5% of the surfaceactive agent since this range gives adequate suspending properties andany amount over 0.5% is superfluous.

The enamel which may be used in this step may be any conventional finishcoat enamel and need not contain any substantial amount ofadherence-promoting oxides. An example of such an enamel is as follows:

Pounds Dehydrated borax 159 Feldspar 143 Sodium nitrate 363 Cryolite 195Zinc oxide 96 Boric acid 345 Powdered quartz 1205 Titanium dioxide 294As a striking example of the unexpected results which can be secured bythe use of our process, a sample of ordinary cold rolled steel, which upto the time of our invention was considered to be practically unusablefor direct finsh coat application, was pickled at room temperature in anitric acid bath comprising one volume of nitric acid to four volumes ofwater; pickling time one minute. The sheet was then rinsed in warm waterand immersed in the chemical acid reduction nickel bath, the analysis ofwhich was given above, at a temperature of 70 C. for one minute. Thesheet was then rinsed again in warm water and dried. By chemicalanalysis the sheet was found to carry a coating of .163 gram of nickelper square foot of surface area.

The prepared sheet was then sprayed with a finish coat porcelain enamel,of the type shown above. The enamel prior to being sprayed on theprepared sheet was milled in the following manner:

Parts by weight Frit 100.0 KCl 0.5 NaAlOz 0.75 Emcol 888 0.032 Water46.0

This slip was then sprayed on the above prepared sheet at a Weight ofabout 20-30 grams (dry components) per square foot and fired at about1520 F. for about 3V2 minutes.

The finished fired sheet had an extremely smooth texture, no surfacepits or copperheads, superior gloss-metal adherence and excellent acidand scratch resistance.

The process comprising our invention may be used to advantage, forexample, in enameling shops and when used makes possible the fabricatingof the desired Work pieces from untreated cold rolled steel which is oneof the lowest cost forms of sheet steel available today.

Other modes of applying the principle of the invention may be employedchange being made as regards the details described, provided thefeatures stated in any of the following claims or the equivalent of suchbe employed.

We, therefore, particularly point out and distinctly claim as ourinvention:

1. In the method of porcelain enameling a cleaned and roughened steelwork piece surface having deep sharp edged pits therein, the stepscomprising immersing said roughened work piece in an aqueous solution ofa nickel salt and depositing from about 0.1 to about 0.2 gram per squarefoot of nickel thereon by the hypophosphite reduction process, whereinthere is no ion exchange between the work piece and said solution,applying thereto a finish coat porcelain enamel clay-free hydrous slipsaid porcelain enamel being substantially free of adherence promotingoxides, said slip further characterized by the inclusion therein of fromabout 0.001% to about 3.0% per 100 parts of frit of a synthetic organiccationic surface active agent and finally firing said porcelain enamelonto said work piece.

2. The method of claim 1 wherein said porcelain enamel IS a titaniumdioxide opacified finish coat porcelain enamel.

References Cited in the file of this patent I Number UNITED STATESPATENTS Name Date Canfield Feb. 25, 1936 Number 8 Name Date Bley Oct.20, 1936 Bley Aug. 24, 1937 McGohan Dec. 14, 1937 Canfield Aug. 16, 1938Van Horn May 23, 1950 Sweo Jan. 1, 1952

1. IN THE METHOD OF PORECLAIN ENAMELING A CLEANED AND ROUGHENED STEELWORK PIECE SURFACE HAVING DEEP SHARP EDGED PITS THEREIN, THE STEPSCOMPRISING IMMERSING SAID ROUGHENED WORK PIECE IN AN AQUEOUS SOLUTION OFA NICKEL SALT AND DEPOSITING FROM ABOUT 0.1 TO ABOUT 0.2 GRAM PER SQUAREFOOT OF NICKEL THEREON BY THE HYDROPHOSPHITE REDUCTION PROCESS, WHEREINTHERE IS NO ION EXCHANGE BETWEEN THE WORK PIECE AND SAID SOLUTION,APPLYING THERETO A FINISH COAT PORECLAIN ENAMEL CLAY-FREE HYDROUS SLIPSAID PORECLAIN ENAMEL BEING SUBSTANTIALLY FREE OF AHDERENCE PROMOTINGOXIDES, SAID SLIP FURTHER CHARACTERIZED BY THE INCLUSION THEREIN OF FROMABOUT 0.001% TO ABOUT 3.0% PER 100 PARTS OF FRIT OF AN SYNTHETIC ORGANICCATIONIC SURFACE ACTIVE AGENT AND FINALLY FIRING SAID PORCELAIN ENAMELONTO SAID WORK PIECE.